The present invention relates to the art of plasma arc torches and, more particularly, to improvements in connection with the starting and re-starting of such torches as well as the preliminary testing and cleaning and the efficient and safe operation thereof.
It is of course well known that a plasma arc torch comprises an electrode and a nozzle which are relatively displaceable between a position in which the electrode contacts the nozzle and a position in which the electrode is spaced an operating distance from the nozzle. The nozzle provides a pilot arc chamber having a plasma outlet opening and, when the electrode contacts the nozzle, an arc current is flowed between the electrode and nozzle, a plasma or arc gas is supplied to the pilot arc chamber, and a pilot arc is created as the electrode moves away from the nozzle to its operating position. As the electrode moves away from the nozzle, it uncovers the plasma outlet opening, whereby a plasma jet is emitted from the nozzle providing for the torch to operate in a non-transferred pilot arc mode. By moving the nozzle into proximity with the workpiece, the arc is transferred thereto and the torch then operates in the arc-transferred mode which is preferred for cutting metal.
Many of arrangements have been provided heretofore for relatively displacing the electrode and nozzle for creating the pilot arc, and a number of such arrangements are shown, for example, in U.S. Pat. Nos. 3,004,189 to Giannini; 3,242,305 to Kane et al; 4,791,268 to Sanders et al; 4,896,016 to Broberg et al; 4,902,871 to Sanders et al; 5,164,569 to Porra et al; and 5,208,441 to Broberg, which patents are incorporated herein by reference for purposes of background information. In the two patents to Sanders et al and the patent to Porra et al, the initial position of the electrode is the position in which the electrode contacts the nozzle and displacement of the electrode from contact with the nozzle to create the pilot arc requires use of the arc gas flowing toward the pilot arc chamber as the primary source of power for displacing the electrode relative to the nozzle. More particularly, in these arrangements the electrode is structurally associated with a piston against which the pressurized arc gas is applied to force the electrode away from the nozzle against the biasing force of a spring by which the electrode is moved into engagement with the nozzle when the arc gas is shut off to de-pressurize the piston chamber. In the patent to Broberg, the initial position of the electrode is that in which the electrode contacts the nozzle, and displacement of the electrode away from the nozzle is achieved through a piston and cylinder arrangement separate from and connected to the electrode through a mechanical linkage arrangement. In Broberg, the supply of arc gas to the torch is tapped to provide the motive fluid for displacing the piston against the bias of a spring which operates through the linkage arrangement to return the electrode into contact with the nozzle when the piston chamber is de-pressurized. In one embodiment shown in Kane et al, the initial position of the electrode is the operative position thereof in which the electrode is spaced from the nozzle, and the electrode is associated with a piston by which the electrode is displaced into contact with the nozzle by applying the arc gas against one side of the piston. Such displacement is against the return bias of a spring by which the electrode returns to its initial position when the piston chamber is de-pressurized. The pilot arc is created during the latter movement of the electrode, and cooling water under pressure is imposed on the opposite side of the piston to hold the electrode in the operative position during operation of the torch. Arrangements of the foregoing character have a number of disadvantages. In this respect, for example, in the patents to Sanders et al and the patents to Porra et al and Broberg at least the arc gas supply if not both the latter and the arc current circuit must be interrupted in order to begin a new pilot cycle. Thus, if the arc is unintentionally extinguished during operation of the torch, the torch cannot be re-started without interrupting arc gas flow. This undesirably increases the cycle time for creating the pilot arc in that, following arc extinction, the operator must wait for the gas supply to shut off and for the power source for the torch to be reactivated and for the gas supply to come back to operating pressure before the pilot arc can be started again. A further disadvantage resides in the fact that, for a realistically sized torch, the available active surface area for the piston is limited such that the spring that forces the electrode against the nozzle must be of a low force. Thus, if the electrode does not touch the nozzle with sufficient force, a pilot arc may not be created because of a build-up of oxides on the electrode and/or nozzle. The limited pressure of the arc gas also limits the latter spring force. Moreover, with the arrangements in the two patents to Sanders et al and the patent to Broberg, it is impossible to have a pre-flow of arc gas such as for cleaning the nozzle-electrode area, or a post-flow of arc gas, such as for cooling the nozzle-electrode area.
In the embodiment of Kane et al referred to, restarting of the torch requires interrupting the power supply in order to evacuate the cooling water from the piston chamber so that the electrode can be displaced into contact with the nozzle. Accordingly, the same undesirably long cycle time attaches to this arrangement in Kane et al. In Broberg et al, a lever and over-center spring mechanism provides for manual displacement of the electrode from its initial position in contact with the nozzle during generation of the pilot arc, and the over-center spring holds the electrode in its operative position. Release of the over-center spring mechanism snaps the electrode back to its position engaging the nozzle and, while the impact of the electrode with the nozzle provides a cleaning action, release of the over-center spring mechanism requires interruption of the torch power supply. Such interruption is also required to intentionally extinguish the arc at the end of a working operation. Therefore, restarting of the torch following unintentional extinction of the arc requires interruption of the power supply and thus an increase in the cycle time between successive starts. Furthermore, it is impossible to have a pre-flow of arc gas prior to initiating generation of the arc, or to have a post-flow of arc gas following an intentional extinction of the arc. Still further, the lever and over-center spring mechanism of Broberg et al, like the lever and piston arrangement of Broberg, require a considerable number of structural components in addition to the electrode, and a disposition of many of the components laterally of the electrode and nozzle axes. Such structural complexity results in an undesirable amount of friction between the moving parts and the potential for the application of side thrust in connection with displacement of the electrode, all of which can affect the ease and reliability of electrode movement.
In Giannini, the electrode in its initial position contacts the nozzle and is spring biased thereagainst, and a solenoid connected to the power source for the torch is operable to displace the electrode away from the nozzle during generation of the arc. Intentional extinction of the arc requires interrupting the power supply whereupon the electrode returns to its initial position engaging the nozzle. Restarting of the torch following an unintentional extinction of the arc also requires interrupting the current source so that the electrode returns to its initial position. Thus, the cycle time for restarting is undesirably high, and there can be no pre-flow or post-flow of arc gas prior to generating the pilot arc or following intentional extinction of the arc.
In another embodiment shown in Kane et al, the electrode is spring biased into contact with the nozzle and cooling water for the nozzle and electrode acts against a piston connected to the electrode to displace the electrode from contact with the nozzle and against the spring bias during generation of the pilot arc. Return of the electrode to its initial position by the biasing spring requires interruption of the power supply in order to reduce the cooling water pressure. Accordingly, restarting of the torch following an unintentional extinction of the arc requires interruption of the power supply and this, together with the time required for reducing the water pressure, results in an undesirably high cycle time between successive starts of the torch. Furthermore, it is not possible to have a pre-flow of arc gas prior to generating the pilot arc or a post-flow following intentional extinction of the arc in that the latter requires interruption of the power supply and thus return of the electrode into contact with the nozzle.
Other problems attendant to the starting and operation of plasma arc torches heretofore available relate to the breaking up and removal of oxide build-up on the electrode and/or nozzle to assure good electrical contact between the latter in connection with generating a pilot arc, and the ability to efficiently, effectively and economically perform a preliminary safety check to assure that the nozzle is in place on the torch and is in proper assembled relationship with the torch body and thus the electrode. Such a safety check is desirable to prevent the operator of the torch from getting accidentally shocked such as by contacting a tool with the electrode if the latter is exposed such as would be the situation if the nozzle is not in place on the torch. At the same time, no or improper contact of the electrode with the nozzle may occur if the nozzle is on the torch but improperly oriented relative to the electrode. The latter can result in the inability to generate a starting arc and/or damage to the nozzle and/or electrode during arc generation as a result of the improper electrical contact therebetween. Heretofore, arrangements for sensing the presence of the nozzle on a torch has required various contacts, springs and the like separate from and in addition to the torch operating components.